The present disclosure relates to a current transformer for a meter, and more particularly, to a power device including a current transformer for minimizing a current measurement error that occurs due to a linearity characteristic difference of the current transformer for a meter and a compensation method for the current transformer.
A current transformer for a meter is applied to any product for measuring and processing a current value. Such a current transformer is mounted or installed on a location where a current is to be measured.
In the current transformer, a part for actually measuring a current and an actual current measured thereby are referred to as a primary side and a primary-side current. Furthermore, a part for transforming the actual current into a processable current and the current obtained by the transforming are referred to as a secondary side and a secondary-side current.
In general, since the primary-side current of the current transformer is a high current that is difficult to be processed, a transformation ratio for transforming the high current is set. Here, the transformation ratio may be a current value ratio between the primary side and the secondary side.
For example, in the case where the value of the current of the primary side, i.e., the value of the actual current, is 400 A and the current value of 400 A is changed to 5 A or 5 V in the secondary side, the transformation ratio is 400:5.
The transformation ratio has a linearity according to a current measurement range of the primary side.
FIG. 1 is a diagram illustrating an operation principle of a typical current transformer, and FIG. 2 is a graph illustrating a linearity characteristic of the typical current transformer.
The principle of measuring a high current of the current transformer will be described with reference to FIGS. 1 and 2. A high current of a primary side induces a magnetic field according to the Ampere's law.
This magnetic flux (Φ) is transferred through an iron core and is interlinked with a secondary side winding so that an electromotive force (E) is induced.
The intensity of the induced electromotive force is based on the Faraday's law of electromagnetic induction, and the direction of the induced electromotive force is determined by the Lenz's law.
That is, the magnetic flux and the electromotive are generated in a direction that offsets a change of the magnetic flux, and, accordingly, a current of a secondary side is generated.
That is, a current transferred to a load is transferred to the primary side of the current transformer, and the current transferred to the primary side is transferred to the secondary side having a value changed according to a turns ratio. Furthermore, a measurement panel is connected to the secondary side, so that the value of the current supplied to the load is measured by reading the value of the current of the secondary side and then applying the turns ratio.
Here, the primary-side current and the secondary-side current are inversely proportional to the turns ratio, and are expressed as the following equation.
                              E          =                      N            ×                                          ⅆ                Φ                                            ⅆ                t                                                    ⁢                                  ⁢                              I            ⁢                                                  ⁢            2                    =                      I            ⁢                                                  ⁢            1            ×                                          N                ⁢                                                                  ⁢                1                                            N                ⁢                                                                  ⁢                2                                                                        (        1        )            
Where, E denotes an electromotive force, I1 denotes a primary-side current, I2 denotes a secondary-side current, N1 denotes the number of turns of a primary side, and N2 denotes the number of turns of a secondary side.
As illustrated in FIG. 2, the current transformer has different linearity characteristics in a low-current region, a middle-current region and a high-current region.
Here, the high-current region, which is outside a region represented by a knee point voltage, is a saturation region where an error of the current transformer increases.
When the primary-side current is increased, the secondary-side current is also increased according to the transformation ratio. However, when arriving at an uppermost limit, the secondary current is saturated and is not increased any more even though the primary current is still increased. At the saturation point, an excitation current becomes 50% when an excitation voltage is increased by 10%, wherein the excitation current is measured by opening the primary winding of the current transformer and increasing an AC voltage with a rated frequency of the secondary winding.
In general, in a saturation characteristic test, an applied voltage at a saturation point is referred to as a saturation voltage. The saturation voltage should be sufficiently high so as to secure protection in the high-current region.
Such current transformers differ with respect to the linearity characteristic even though the current transformers are identical products manufactured by the same manufacturer.
Therefore, current transformers undergo a test in order to be applied to a system. The test may be classified into a shop test and a field test.
The shop test includes a type test performed on sample products in order to check and verify the characteristics of current transformers and a routine test performed on all products in order to evaluate the performance of all products.
After evaluating the performance of individual products through the shop test, the products are applied to a system so as to undergo the field test through procedures such as a component test and a linking test.
The current transformers as described above are applied to a large-scale power system such as a high voltage direct current (HVDC) transmission system or a product or system for transforming power. Such systems are designed so as to measure current at multiple places to control or protect the systems.
The current transformers applied as described above have different linearity characteristics according to a current range even though the current transformers are identical products. Although a verification test is performed to test the current transformers applied to a product or a system, a problem due to a linearity difference may highly possibly occur in a system in which the transformation ratio is high or two or more current transformers are applied.
In this case, even though a current of the same route is measured at multiple places, a current measurement error may occur due to a difference of linearity characteristics of current transformers installed on the places. Such an error may be recognized as system failure, may cause generation of an alarm, or may even cause an interruption of system operation.
In addition, since the current transformers as described above have different linearity characteristics, one measurement panel should be connected to one current transformer, causing an increase of the unit cost of products.